Since then, Mr. Simiyu, 41, has painstakingly been building his case. Trawling records from the phone company, running surveillance on suspects, and partnering closely with the bank’s own investigations team, today he’s close to an arrest.

“These kinds of electronic crimes, they are something new, something I’ve only really started seeing in the last four or five years,” he told me earlier, in his dimly-lit office at Nairobi’s Criminal Investigations Division headquarters.